The present invention relates to manipulation of materials dissolved in solvents. More specifically, in one embodiment, the invention provides an improved continuous flow interface for a liquid chromatograph.
Liquid chromatography systems and their use are well known to those of skill in the art. In a liquid chromatography system, a mixture of materials is separated for analysis. The mixture is dissolved in a suitable solvent and injected into the top of a column packed with a suitable adsorptive material. As the material flows through the column, the various materials are adsorbed to the packing at different rates such that the eluents emerging from the bottom of the column are spatially separated with the less highly adsorbed materials generally leaving first and the more highly adsorbed materials leaving later.
It is often desirable to remove the solvent from the separated components, since the solvent can interfere with post-separation work with the components. This work might involve chemical or instrumental analysis, or chemical reactions of the components. For example, it is often desirable to flow the effluent from the column into an IR spectrometer for analysis. One set of techniques has involved passing the effluent from the chromatograph through a flow cell and measuring the infrared transmission spectra of the separated components. One problem with this technique is that the solvent typically has an absorption spectrum that must somehow be subtracted out of the measured spectra. Depending upon the particular materials and solvent involved, this is sometimes difficult or impossible to fully resolve. An alternative process for obtaining spectra involves removing the solvent from the effluent and taking spectra of the residual sample materials.
A variety of techniques for elimination of part or all of the LC solvent in a LC effluent have been proposed, some of which are described in Griffiths et al., "Solvent Elimination Techniques for HPLC/FT-IR," incorporated by reference herein for all purposes. It is often desirable to provide material to such devices at relatively constant flow rates or solvent composition. This problem has been difficult to resolve. Another technique, which has been proposed for use as a concentrator in a liquid chromatography mass spectrometry interface, is described in White et al., U.S. Pat. No. 4,281,246. This technique provides for a system in which LC solvent flows down a heated wire. As the effluent flows down the wire, solvent is preferentially evaporated, resulting in an effluent which is more highly concentrated in the materials of interest.
While meeting with substantial success, certain problems remain with the system of White et al. for removing solvent from a LC effluent. For example, (1) band broadening: time resolution of the LC peaks is not maintained; (2) spacial broadening: material is spread into too large an area; (3) the effluent has insufficient concentration.
One source of these problems is the inability to achieve an uniform flow rate through interface. The difficulties in non-uniform flow stem, in part, from the nature of the wire guides used in the interfaces. First, these wire guides have problems with wetability, preventing the liquid from forming an even coat over the guide. Second, using the wire guide itself as a heat source causes it to become hotter than the surrounding liquid. When the liquid reaches its boiling point, it evaporates off the wire and leaves dry spots. Without liquid to cool the wire, the temperature of the dry spot increases further. When liquid hits one of these local dry spots it immediately sputters off, making it impossible to obtain a uniform liquid coat over the surface. The uniform flow problem is further exacerbated by the non-uniform multiple stages of the wire described in the prior art. These guide wires have stages with decreasing diameter and resistance. At the transition points between stages, the liquid tends to form beads. These difficulties with obtaining a uniform flow over the guide limit the minimum flow rate achievable, and consequently, decrease the concentration that the wire guide based interfaces can achieve.
Another problem with the prior art concentrator system of White et al. is the feedback control system. The response of the drop-size monitor is too slow to be effective when the solvent changes in composition, as commonly occurs during solvent programming in liquid chromatography.
From the above it is seen that an improved sample concentrator between a liquid chromatograph and a second system such as an IR analysis device is needed.